ULTRA-MICROSCOPIC OR FILTERABLE VIRUSES 245 



ranging from 55 to 70 C. Some resist drying for com- 

 paratively long periods of time, others are quickly killed by 

 it. Practically all are resistant to the action of glycerin. 

 This is not the case as a rule with bacteria. They vary 

 considerably in their resistance to such germicidal substances 

 as formalin, boric acid, corrosive sublimate and menthol. 



Practically all animals that survive their invasion have 

 acquired immunity from a second attack of the disease. 

 There is little evidence that the growth is accompanied by 

 the production of toxins as such. A survey of such data as 

 are available justifies the suspicion that these bodies are 

 more closely allied to the protozoa than to the bacteria. 



Efforts at cultivation under artificial circumstances have 

 succeeded in only a few instances. In their studies upon the 

 contagious pleuro-pneumonia of cattle Nocard and Roux 

 by the use of special methods, both optical and cultural, 

 claim to have demonstrated the causative factor of that 

 disease. The method employed by them for the cultivation 

 of the virus is that suggested by Metchnikoff,* Roux and 

 Salambini in 1896. It consists in placing bits of tissue or 

 secretions from the infected animals in small, sterilized 

 collodion sacs, which are finally hermetically sealed with 

 sterile collodion. These little sacs with their contents are 

 then placed in the peritoneal cavity of an animal; a rabbit, 

 chicken, guinea-pig, calf, dog, or sheep as the case may be, 

 and left there for a time. The idea on which this method is 

 based is that the collodion sacs are impermeable for the 

 specific virus but are permeable to the normal juices of the 

 peritoneal cavity of the animal in which they are placed. 

 Under these circumstances the specific virus was expected to 

 develop within the sacs and receive its food supply by dif- 

 fusion from the surrounding peritoneum; the body tern- 



